PbSe is an attractive tellurium-free thermoelectric material for the intermediate temperature power generation. However, the performance of PbSe is much poorer than its more expensive isostructural analogue of PbTe. Here we present that PbSe0.998Br0.002-2%Cu2Se exhibits peak ZT 1.8 at 723 K and average ZT 1.1 between 300 and 823 K, which are the highest values to date for all previously reported n- and p-type PbSe-based materials as well as for tellurium-free n-type polycrystalline materials. Cu2Se doping simultaneously flattens the conduction band edge of PbSe, increasing the effective mass of charge carriers, and band gap, which substantially improves the Seebeck coefficient. It also raises the carrier concentration with a negligible damage in carrier mobility, thereby even improving the electrical conductivity above ∼423 K. These induced factors synergistically result in ultrahigh power factor, reaching ∼21−26 μW cm−1 K−2 over a wide range of temperature from ∼423 to 723 K. Cu2Se doping markedly depresses the lattice thermal conductivity to ∼0.4 W m−1 K−1 at 773 K, which is close to its theoretical amorphous limit. First-principles calculations show that the obtained ultralow value originates from the remarkable softening of acoustic phonon at the low-frequency region.